1. Field of the Invention
The present invention relates to variable resistors and in particular to a ceramic resistor card for use in a transducer for converting a physical position into an electrical signal with interlaced glass fingers for longer wear from wiper contacts.
2. Description of the Related Art
Variable resistors are known for sensing parameters in a variety of applications. For example, the fuel level in an automobile tank is typically measured using a variable resistor having a sweep arm mechanically or electrically coupled to a float located in a fuel tank. The sweep arm position is determined by the level of fuel in the tank and the voltage value sensed across the variable resistor depends on the position of the sweep arm.
The variable resistor typically comprises a resistor card with metalized areas and thick film resistive ink. The thick film ink is deposited in precise areas to interconnect respective metalized areas. There is a wiper blade assembly having contact fingers thereon connected to the sweep arm. As the float raises and lowers according to the fluid level, the wiper contact fingers move along the resistor card in an arcuate path and make corresponding contact with the metalized areas. This results in a voltage change that generates a signal representative of the amount of fuel contained in the tank.
One problem with this type of system is that over the life of a vehicle it must go through thousands of cycles in a harsh environment. The card is often exposed to both fuel and dry conditions as the fluid level is decreased. In addition, the assembly is exposed to severe vibration resulting from the vehicle movement. The result is that wear occurs as the wiper contact fingers go back and forth over the metalizations and the electrical resistance of the metalization portions may increase and cause accuracy problems with the fuel reading. Additionally, the metalized portions may wear to the point that an open circuit results along some given or several metalization locations.
Specifically, there are problems when conductors lines are broken up into finger-like projections or short conductor lines with spaces therebetween. When a wiper blade assembly is rubbed over the spaced apart conductor lines, the edge of each conductor line begins to be worn down from repetitive impact. Additionally, the wiper blade assembly contact fingers are equally worn down faster because of the repetitive impact with the edges of the conductor lines. Further wear is encouraged by the contact fingers bouncing up and down as they travel approximately perpendicular across the orientation of the conductor line sets.
3. Related Art
Examples of patents related to the present invention are as follows, and each patent is herein incorporated by reference for the supporting teachings:
U.S. Pat. No. 3,613,042, is a variable resistance element with spaced rows of parallel tabs.
U.S. Pat. No. 4,032,881 is a resistance element with improved linearity and method of making the same.
U.S. Pat. No. 4,318,075 is a thick film potentiometer having a wiper track with conductor lines deposited in a parallel fashion.
U.S. Pat. No. 4,500,866 is a nonlinear potentiometer including an arcuate resistor pad connected to a plurality of parallel resistor pads by means of a plurality of conductor lines.
U.S. Pat. No. 4,931,764 is a low wear resistance card for use in a liquid fuel sender card.
U.S. Pat. No. 5,051,719 is a thick-film non-step resistor with accurate resistance characteristics.
U.S. Pat. No. 5,169,465 is a thick-film circuit element on a ceramic substrate.
The foregoing patents reflect the state of the art of which the applicant is aware and are tendered with the view toward discharging applicants' acknowledged duty of candor in disclosing information that may be pertinent in the examination of this application. It is respectfully stipulated, however, that none of these patents teach or render obvious, singly or when considered in combination, applicant's claimed invention.